filters. For the pre-concentration, the capacity is from 1J^ to 2 gal. per square foot in a horizontal-tube evaporator, and for the crystallization a vertical-tube machine is used with a capacity of about 1 gal. per square foot. The steam pressure is 5 lb., and the vacuum 27 in. Evaporators are built with cast-iron shells and steel or wrought-iron tubes.
Sodium Benzol Sulphonate.—These solutions are concentrated from 10 to 50 per cent solids in double or triple effects of the horizontal-tube, vertical-tube, or rapid-circulation type, with a capacity of from 2 to 2^ gal. per square foot, at a steam pressure of 5 lb. and a vacuum of 27 in. Cast-iron or steel shells with wrought-iron or steel tubes is common practice.
Glycerin.—In the soap industry, 1,000 lb. of fat will give about 2,000 lb. of soap lyes containing 5 per cent glycerin and about 10 per cent of salts. These liquors are usually concentrated in a double- or triple-effect evaporator of the horizontal-tube or vertical-tube type to from 40 to 50 per cent of glycerin. With a steam pressure of 5 lb. and a vacuum of 27 in.; the capacity is about 1 gal. per square foot. The 50 per cent solution is further concentrated to about 80 per cent in a single effect.
In the candle and oil industry, large quantities of glycerin are produced by the Twitchell process. The liquors are usually concentrated from 15 to 80 per cent in single-effect or double-effect evaporators of the vertical-tube, horizontal-tube or rapid-circulation type. The capacity is from 1J^ to 2 gal. per square foot with a steam pressure of 5 lb. and a vacuum of 28 in. All glycerin solutions should be handled in evaporators with special high-vapor space and large catchalls or entrain-ment separators to prevent losses by foaming. Glycerin produced by the Twitchell process will form a hard scale on the tubes, consisting mostly of calcium sulphate.
Grape Juice and Cider.—The initial density of these liquors will vary from 6 to 8°Be"., and the final concentration is usually carried to from 30 to 33°Be*. The capacity is from 1 y% to 2 gal. per square foot with a steam pressure of 5 lb. and a vacuum of from 26 to 28 in. Only single effects can be used in order to avoid high temperature and a cooked taste in the liquor. The evaporators must be built of copper, which is sometimes tinned. Machines of the vertical-tube or rapid-circulation type are commonly used, but a number of factories are also using the ordinary coil pan with shells made of copper or enamel steel.
RECOVERY OF WASTE LIQUORS
Black Liquor from Soda-pulp Mills.—One ton of pulp will produce about 3,300 gal. of 5 deg. liquor, and this is concentrated to 35 deg. in multiple-effect evaporators of the horizontal and film type, with a capacity of from 2 to 2H gal. per square foot, according to the steam pressure, which will be from 10 to 25 lb., with a vacuum of from 27 to 28 in. Special construction and separators are necessary on account of the excessive foaming. Evaporators must have cast-iron or steel shells with wrought-iron or steel tubes, as the liquors are strongly alkaline.
Sulphate Liquor.—One ton of sulphate pulp which is used for the manufacture of Kraft paper produces about 2,000 gal. of liquor which is to be concentrated from 10 to 20°Be*. Usually a double or triple effect of the horizontal type is used for this purpose, and the capacity is from 2 to 2>£ gal. per square foot according to the steam pressure. The concentration of this 20°B6, liquor is continued